Add calendar export, SEO, error monitoring, backups, audit trail, and a starter test suite

Six site improvements picked from a "what could be better" review, plus a Jest
test suite covering the two areas with the trickiest money-handling history
in this project (early-bird pricing tranches, donation-leg accounting):

- "Add to calendar" .ics download on event pages and in confirmation emails
- sitemap.xml, robots.txt, and Open Graph/Twitter metadata for public pages
- Sentry error monitoring (backend + frontend), a no-op until SENTRY_DSN is set
- Nightly local pg_dump backups with a Site Settings tab to browse/trigger/download
- Admin audit trail for refunds, donations, manual registrations, event and
  settings changes, and staff-initiated cancellations
- Jest tests reproducing and guarding against the 1.8.0 tranche-pricing bug
  and the 1.4.2 donation-balance-inflation bug

Wallet passes (Google/Apple) were scoped out of this round — Apple Wallet
needs a paid Apple Developer account the project doesn't have yet, and the
user preferred shipping both together later rather than Google alone now.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/instrumentation.ts

@@ -0,0 +1,17 @@
+// Next.js server instrumentation entry point. A no-op when NEXT_PUBLIC_SENTRY_DSN isn't
+// set — see instrumentation-client.ts for the browser-side counterpart.
+//
+// Node runtime only, deliberately — this app's middleware.ts (which runs in Next's edge
+// runtime) is a trivial pass-through with no real error surface, so an edge-runtime branch
+// here would only inflate that shared middleware bundle for no actual coverage benefit.
+export async function register() {
+  if (!process.env.NEXT_PUBLIC_SENTRY_DSN) return;
+  if (process.env.NEXT_RUNTIME !== "nodejs") return;
+
+  const Sentry = await import("@sentry/nextjs");
+  Sentry.init({
+    dsn: process.env.NEXT_PUBLIC_SENTRY_DSN,
+    environment: process.env.NODE_ENV || "development",
+    tracesSampleRate: 0.1,
+  });
+}